The present invention relates to a shift control system of a hybrid transmission which is applicable to a hybrid vehicle equipped with a motor/generator and a prime mover such as an internal combustion engine, and more particularly to a control system of a continuously variable hybrid transmission which is capable of continuously varying a transmission ratio using a differential mechanism disposed between the motor/generator and the prime mover.
U.S. Pat. No. 6,190,283 (≅ JP-A-2000-236602) discloses a hybrid transmission system for a hybrid vehicle equipped with a prime mover, a generator in direct connection with the prime mover and a driving motor receiving electric power from the generator. The generator is controlled in speed by executing a revolution speed control of the prime mover, and the driving motor is controlled in torque so as to generate a target driving force. In this hybrid transmission system, the generator is directly connected to the prime mover, and the driving motor is separated from the generator and the prime mover. Therefore, the operating point of the driving motor does not affect the optimal operating point of the prime mover, and it is easy to set both of the optimal operating points. However, since this hybrid transmission is of a motor direct connection type, a current passing through the direct-connection motor and the inverter becomes large. Therefore, the loss of the direct-connection motor and the inverter becomes large, and the direct-connection motor and the inverter are required to have a large capacity.
In order to solve this problem, an applicant of the present invention has proposed a hybrid transmission which is constructed by providing a two-degree-of-freedom differential mechanism between a prime mover and a motor although it has not been commonly disclosed yet, and rotating members of the differential mechanism are connected to the prime mover, a driveline and two motor/generators, respectively, so that a continuously variable transmission control is executed by the control of the motor/generators.
However, the inventor of the present invention has found that such a proposed hybrid transmission has been yet required to be further improved in operation. More specifically, this proposed hybrid transmission is designed such that if a target prime-mover revolution speed for achieving a target driving force is derived from a driving condition while maintaining the transmission output revolution speed (vehicle speed), the revolution speeds of two motor/generators are inevitably determined. If the revolution speed of one of motor/generators is high, the cupper loss of the motor/generator also becomes high due to the excessive weakened magnetic field, and therefore the motor/generator and the inverter generate a large quantity of heat due to the cupper loss. Accordingly, even if an operating point of the engine is determined at an operating point as a combination of a revolution speed and an output torque for generating the target driving force without changing the transmission output revolution speed, the inevitably determined operating points of the motor/generators may not be the operating point optimal for the motor/generators, and there is a possibility that the motor/generators and the inverter generates a lot of heat due to the large cupper loss.
In order to eliminate such excessive heat generation, it is necessary to provide a heavy-duty cooling system for the motor/generator and the inverter. However, such a heavy-duty cooling system increases the difficulty in design and in production cost of the system.
It is therefore an object of the present invention to provide an improved control system for a hybrid transmission which monitors a heat generating condition of each of motor/generators and an inverter, and changes the target prime-mover revolution speed when the temperature of the monitored objects is raised, to lower the generated power of the motor/generator and thereby suppressing the heat generation so as to enable the system to operate without a heavy-duty cooling system.
An aspect of the present invention resides in a control system of a hybrid transmission for a hybrid vehicle. The hybrid transmission employs a two-degree-of-freedom differential mechanism comprising at least four rotating members, the differential mechanism determining rotating conditions of all of the rotating members when rotating conditions of two of the rotating members are determined. The rotating members are connected to a prime mover, a driveline and first and second motor/generator, respectively. The control system comprises a controller which is configured to change a target prime-mover revolution speed of the prime mover so as to decrease an electric power output of the first and second motor/generators when a temperature of one of the first and second motor/generators and a power device for the first and second motor/generators is higher than a predetermined temperature, wherein a target driving force of the hybrid vehicle being achieved by a demand prime-mover output generated when an output revolution speed of the hybrid transmission is kept constant and when the target prime-mover revolution speed is determined.
Another aspect of the present invention resides in a hybrid transmission system for a hybrid vehicle equipped with a prime mover and first and second motor/generators. The hybrid transmission system comprising: a two-degree-of-freedom differential mechanism comprising at least four rotating members, rotating conditions of all of the rotating members are determined when rotating conditions of two of the rotating members are determined, the rotating members being connected to the prime mover, a driveline and the first and second motor/generators, respectively; a temperature sensor for detecting temperatures of the first and second motor/generators and a power device for the first and second motor/generators; a prime-mover controller for controlling a prime-mover operating point defined by a revolution speed and an output torque of the prime mover; a motor/generator controller for controlling each motor/generator operating point defined by a revolution speed and an output torque of each of the first and second motor/generators; and a hybrid controller coupled to the temperature sensor, the prime-mover controller and the motor/generator controller, the hybrid controller being configured to change a target prime-mover revolution speed to be sent to the prime-mover controller so as to decrease an electric power generated by the first and second motor/generators when one of the temperatures detected by the temperature sensor is higher than a predetermined temperature, wherein the prime mover generates a demand output for achieving a target driving force when a transmission output revolution speed indicative of a revolution speed of the driveline is kept constant and when the prime motor operates at a prime-mover operating point determined by the target prime-mover revolution speed and an output torque of the prime mover.
A further another aspect of the present invention resides in a method of controlling a hybrid transmission for a hybrid vehicle. The hybrid transmission employs a two-degree-of-freedom differential mechanism comprising at least four rotating members, the differential mechanism determining rotating conditions of all of the rotating members when rotating conditions of two of the rotating members are determined. The rotating members is connected to a prime mover, a driveline and first and second motor/generator, respectively. The method comprising: changing a target prime-mover revolution speed of the prime mover so as to decrease an electric power output of the first and second motor/generators when a temperature of one of the first and second motor/generators and a power device for the first and second motor/generators is higher than a predetermined temperature, a target driving force of the hybrid vehicle being achieved by a demand prime-mover output generated when an output revolution speed of the hybrid transmission is kept constant and when the target prime-mover revolution speed is determined.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.